exceptions were found to be acid; and the exceptions may have been
due to such castings not having been recently ejected; for some
which were at first acid, were on the following morning, after
being dried and again moistened, no longer acid; and this probably
resulted from the humus acids being, as is known to be the case,
easily decomposed. Five fresh castings from worms which lived in
mould close over the chalk, were of a whitish colour and abounded
with calcareous matter; and these were not in the least acid. This
shows how effectually carbonate of lime neutralises the intestinal
acids. When worms were kept in pots filled with fine ferruginous
sand, it was manifest that the oxide of iron, with which the grains
of silex were coated, had been dissolved and removed from them in
the castings.

The digestive fluid of worms resembles in its action, as already
stated, the pancreatic secretion of the higher animals; and in
these latter, "pancreatic digestion is essentially alkaline; the
action will not take place unless some alkali be present; and the
activity of an alkaline juice is arrested by acidification, and
hindered by neutralization." {27} Therefore it seems highly
probable that the innumerable calciferous cells, which are poured
from the four posterior glands into the alimentary canal of worms,
serve to neutralise more or less completely the acids there
generated by the half-decayed leaves. We have seen that these
cells are instantly dissolved by a small quantity of acetic acid,
and as they do not always suffice to neutralise the contents of
even the upper part of the alimentary canal, the lime is perhaps
aggregated into concretions in the anterior pair of glands, in
order that some may be carried down to the posterior parts of the
intestine, where these concretions would be rolled about amongst